Project 3 Build an Astable Multivibrator

1. Project 3Build an Astable Multivibrator

2. Purpose • The purpose of this project is to build an Astable multivibrator without the 555-timer chip. • This means you will have to assemble your own components to mimic the behavior of the inside of the chip. • You will create a PSpice simulation and a working circuit. • You will then modify the 555 timer chip model so that it cycles over a different part of the capacitor charge curve. • You will modify your PSpice simulation and circuit to demonstrate that your new model works as predicted. Electronic Instrumentation

3. The Animation Animation applet Your initial design will be a PSpice simulation and working circuit based on this animation. Electronic Instrumentation

4. Block Diagram • Circuits are often represented by block diagrams that show the flow of the signal between different functional blocks. • Above is a block diagram of the astable multivibrator. • Your circuit won’t include the Reset feature Electronic Instrumentation

5. Components in each Block A G E C D F Electronic Instrumentation B

6. Components in each Block A: R-R-C Combination B: Voltage Divider C: Threshold Comparator D: Trigger Comparator E: Reset Logic Chip (NAND gate) F: SR Flip Flop G: Transistor Circuit Electronic Instrumentation

7. How does the Astable Multivibrator work? What makes this circuit generate a string of pulses? This is discussed in detail in the experiment 7 notes. http://www.academy.rpi.edu/5.downloads/modules.html Electronic Instrumentation

8. How does the Astable Multivibrator work? 2 NOR gates can be used to create a SR Flip-Flop Convince yourself that this works Electronic Instrumentation

9. Build this circuit This has all the 555 Timer features except for the reset pin. You will build it on the protoboard Electronic Instrumentation

10. But model this circuit The demo version of Capture won’t model the circuit you will build. It can model this one, which uses 2 transistors to model the SR Flip-flop. Electronic Instrumentation

11. How does the Astable Multivibrator work? These equations determine the characteristics of your output pulses based on the values you choose for R1, R2 and C1. Electronic Instrumentation

12. How does the Astable Multivibrator work? • The frequency of the pulses and their duty cycle are dependent upon the RC network values. • The capacitor C charges through the series resistors R1 and R2 with a time constant of tON = (R1 + R2)C1. • The capacitor discharges through R2 with a time constant of tOFF = R2C1 Electronic Instrumentation

13. Where do the equations come from? The equations that determine the on and off time of the output pulses are based on the charge and discharge time of the capacitor. The capacitor equations are: charging discharging Electronic Instrumentation

14. Relating charge equations to time How much time should it take to charge between 1/3 and 2/3 of V0? Time to charge up to 2/3V0 is: Electronic Instrumentation

15. Initial Design PSpice • Build the PSpice circuit and look at the signals at the input and output of each block in the diagram. • No reset circuit • Use the cursors to record voltage levels and times • high and low on digital signals • important points on analog signals (like 1/3 and 2/3 of Vcc) • on and off time of the pulses Electronic Instrumentation

16. Initial Design Protoboard • Build the circuit on your protoboard • don’t forget to put power on the digital chip • add a bypass capacitor • Record data using Mobile Studio and the IOBoard • Use voltage and time features of scope • Use the cursors on the scope • Make sure you have labeled the plots with the numerical values recorded Electronic Instrumentation

17. Final Design • Modify the inside of the timer to make it switch at different voltages. • What are the new equations for TON and TOFF? • What are the new on and off times for the pulses in your circuit? • Modify the PSpice and the circuit on your protoboard and show that your results are consistent with those predicted by the equations. Electronic Instrumentation

18. Project Report • Introduction • What is the objective of the project? • At least two relevant topics • Theory • Describe the function of the components in the circuit • How does the multivibrator work? Give details. • Where do the equations for TON and TOFF come from? • What should TON and TOFF be for the circuit you are building? Electronic Instrumentation

19. Project Report • Initial Design • PSpice simulation, plots, and discussion • Protoboard implementation, plots, and discussion • comparison of voltages and times • PSpice • Protoboard • Theory Electronic Instrumentation

20. Project Report • Final Design • Determine new threshold and trigger voltages • Come up with the new timing equations • Modify PSpice • Modify Circuit • Comparison of voltages and times • voltage levels affected by redesign • new on and off times Electronic Instrumentation

21. Project Report • Conclusion • Is it an astable multivibrator? • Conclusions that can be drawn from your voltage comparisons • Discuss the on and off times of the initial and final design. Are they as expected? • Sources of error • General Conclusions Electronic Instrumentation

22. Appendices • Appendix A: Make you own task list. • Appendix B: References and initial design equations. • Appendix C: PSpice plots of initial design • Appendix D: Plots of data from Mobile Studio for initial design • Appendix E: Final design (circuit diagram, calculations, PSpice and Mobile Studio plots) Electronic Instrumentation